1. Field of the Invention
This invention relates generally to touch sensor technology. Specifically, the invention is related to a method of reducing noise in a capacitive touch sensor using mutual capacitance technology to detect and track conductive objects in contact with and/or in proximity to the touch sensor, and wherein the system also includes the ability to perform de-ghosting in order to determine the actual location of the conductive objects.
2. Description of Related Art
It is useful to describe prior art touch sensor, touchpad and touchscreen technology that can be modified to use the present invention (hereinafter referred to only as “touchpad”). Specifically, the capacitance-sensitive touchpad and touchscreen technology of CIRQUE® Corporation can be modified to implement the present invention. The CIRQUE® Corporation touchpad is a mutual capacitance-sensing device and an example is illustrated in FIG. 1. The touchpad can be implemented using an opaque surface or using a transparent surface. Thus, the touchpad can be operated as a conventional touchpad or as a touch sensitive surface on a display screen, and thus as a touch screen.
In this touchpad technology of Cirque® Corporation, a grid of row and column electrodes is used to define the touch-sensitive area of the touchpad. Typically, the touchpad is a rectangular grid of approximately 16 by 12 electrodes, or 8 by 6 electrodes when there are space constraints. Interlaced with these row and column electrodes is a single sense electrode. All position measurements are made through the sense electrode. However, the row and column electrodes can also act as the sense electrode, so the important aspect is that at least one electrode is driving a signal, and at least a different electrode is used for detection of a signal.
In more detail, FIG. 1 shows a capacitance sensitive touchpad 10 as taught by CIRQUE® Corporation includes a grid of row (12) and column (14) (or X and Y) electrodes in a touchpad electrode grid. All measurements of touchpad parameters are taken from a single sense electrode 16 also disposed on the touchpad electrode grid, and not from the X or Y electrodes 12, 14. No fixed reference point is used for measurements. Touchpad sensor control circuitry 20 generates signals from P, N generators 22, 24 (positive and negative) that are sent directly to the X and Y electrodes 12, 14 in various patterns. Accordingly, there is typically a one-to-one correspondence between the number of electrodes on the touchpad electrode grid, and the number of drive pins on the touchpad sensor control circuitry 20. However, this arrangement can be modified using multiplexing of electrodes.
The touchpad 10 does not depend upon an absolute capacitive measurement to determine the location of a finger (or other capacitive object) on the touchpad surface. The touchpad 10 measures an imbalance in electrical charge to the sense line 16. When no pointing object is on the touchpad 10, the touchpad sensor control circuitry 20 is in a balanced state, and there is no signal on the sense line 16. There may or may not be a capacitive charge on the electrodes 12, 14. In the methodology of CIRQUE® Corporation, that is irrelevant. When a pointing device creates imbalance because of capacitive coupling, a change in capacitance occurs on the plurality of electrodes 12, 14 that comprise the touchpad electrode grid. What is measured is the change in capacitance, and not the absolute capacitance value on the electrodes 12, 14. The touchpad 10 determines the change in capacitance by measuring the amount of charge that must be injected onto the sense line 16 to reestablish or regain balance on the sense line.
The touchpad 10 must make two complete measurement cycles for the X electrodes 12 and for the Y electrodes 14 (four complete measurements) in order to determine the position of a pointing object such as a finger. The steps are as follows for both the X 12 and the Y 14 electrodes:
First, a group of electrodes (say a select group of the X electrodes 12) are driven with a first signal from P, N generator 22 and a first measurement using mutual capacitance measurement device 26 is taken to determine the location of the largest signal. However, it is not possible from this one measurement to know whether the finger is on one side or the other of the closest electrode to the largest signal.
Next, shifting by one electrode to one side of the closest electrode, the group of electrodes is again driven with a signal. In other words, the electrode immediately to the one side of the group is added, while the electrode on the opposite side of the original group is no longer driven.
Third, the new group of electrodes is driven and a second measurement is taken.
Finally, using an equation that compares the magnitude of the two signals measured, the location of the finger is determined.
Accordingly, the touchpad 10 measures a change in capacitance in order to determine the location of a finger. All of this hardware and the methodology described above assume that the touchpad sensor control circuitry 20 is directly driving the electrodes 12, 14 of the touchpad 10. Thus, for a typical 12×16 electrode grid touchpad, there are a total of 28 pins (12+16=28) available from the touchpad sensor control circuitry 20 that are used to drive the electrodes 12, 14 of the electrode grid.
The sensitivity or resolution of the CIRQUE® Corporation touchpad is much higher than the 16 by 12 grid of row and column electrodes implies. The resolution is typically on the order of 960 counts per inch, or greater. The exact resolution is determined by the sensitivity of the components, the spacing between the electrodes on the same rows and columns, and other factors that are not material to the present invention.
Although the CIRQUE® touchpad described above uses a grid of X and Y electrodes and a separate and single sense electrode, the sense electrode can be eliminated and the function of the sense electrode is the set of electrodes not being used to drive a signal.
The method of using a single sense electrode has provided a very simple/low cost solution by requiring only a single Rx channel. The single sense line, however, creates measurements that are susceptible to noise even for measurement patterns not near a finger. The noise in the measurements for electrode patterns not near the finger cause the extraction method to falsely detect finger presses in random positions all over the surface which result in false positive gestures, cursor jitter and gesture state machine mode confusion manifesting in unresponsive cursor movement.
It is desirable to use the mutual capacitance technology of CIRQUE® Corporation in order to provide multi-touch capabilities. It would be an advantage to eliminate the sense line, use de-ghosting in order to determine the actual location of all the fingers, and modify the existing technology in order to reduce sensitivity to noise.